The present invention relates to a dielectric thin film of a substituted lead titanate. More particularly, the invention relates to a dielectric thin film of a substituted lead titanate capable of exhibiting a high dielectric constant even when the thickness of the thin film is very small to be 1 .mu.m or smaller.
As is well known, lead titanate of the chemical formula PbTiO.sub.3 is a typical ferroelectric material having a crystallographic structure of perovskite. Thin films of lead titanate are promising as a material for infrared sensors, microactuators, memories and the like and extensive investigations are now underway for the development of these applications. One of the serious problems in the application of a thin film of lead titanate as a dielectric material is that the dielectric properties of the thin film depend on the thickness of the thin film and, in particular, the dielectric constant of a thin film of lead titanate is greatly decreased as compared with the value for a bulk of the same material when the thickness of the thin film is 1 .mu.m or smaller. A report in this regard is found, for example, in Proc. FMA-5, page 7 (1985), according to which the dielectric constant of a thin film of lead titanate formed on a substrate by the sol-gel method is only one third to one fourth of the value in bulk when the thin film has a thickness of 1 .mu.m or smaller.
Various proposals and attempts have been made heretofore with an object to provide a lead titanate-based dielectric thin film which does not suffer from a decrease in the dielectric constant when the thickness thereof is decreased including the approaches for the modification of the preparation method of the thin film and introduction of some additive or substituting elements into the crystalline structure of lead titanate. For example, it is reported in Journal of Applied Physics, volume 61, pages 411-415 (1987) and elsewhere that an improvement in the dielectric characteristics can be obtained when a thin film is formed from so-called PLT of (Pb,La)TiO.sub.3, which is a lead titanate substituted by lanthanum atoms at a part of the lead atom positions in the crystal lattice of PbTiO.sub.3. The improvement obtained by PLT, however, is still insufficient not to impart practicability to lead titanate-based dielectric thin films.